1.8.
Weather forecasting process,
observations, analyses and forecasts
including
graphical
representation
of
GDPFS Centres that have a mandate of chart-based analysis shall maintain standardized
weather forecasting process, including graphical representation of observations, analyses
and forecasts.
1.8.1 STANDARD SETS OF GRAPHICAL REPRESENTATION
1.8.1.1 Standard set of symbols and styles
A standard set of symbols and styles should be used in graphical representations of
observations and analyses. This may also be used for presentation of forecasts. This set is
given in XXX of the Manual on Coders (WMO-No.306) (Appendix II-4 of the current GDPFS
Manual is planned to be incorporated into the Manual on Codes.)
1.8.1.2 Colour scales for colour maps
All colour scales for colour maps should be properly defined to avoid misinterpretation by any
category of users, and they should be standardised whenever possible.
1.8.2 ANALYSIS AND FORECASTING PRACTICES
1.8.2.1 Reference surfaces for upper-air analysis
1.8.2.1.1 The principal type of reference surface for representing and analysing the
conditions in the free atmosphere over large areas shall be isobaric.
1.8.2.1.2 The standard isobaric surfaces for representing and analysing the conditions in the
lower atmosphere shall be the 1 000 hPa, 850 hPa, 700 hPa, 500 hPa, 400 hPa, 300 hPa,
250 hPa, 200 hPa, 150 hPa and 100 hPa surfaces.
1.8.2.1.3 The standard isobaric surfaces for representing and analysing the conditions in the
atmosphere above 100 hPa should be the 70 hPa, 50 hPa, 30 hPa, 20 hPa and 10 hPa
surfaces.
1.8.2.2 Preparation of upper-air charts
1.8.2.2.1 Centres should either prepare or have available upper-air charts for at least four of
the six following standard isobaric surfaces: 850 hPa, 700 hPa, 500 hPa, 300 hPa, 250 hPa
and 200 hPa.
1.8.2.3 Weather forecasting
The weather forecasting process includes analysis, prognoses and interpretation of state and
forecasts of weather parameters at the surface and/or in the free atmosphere for short,
medium, extended and long ranges for purposes according to the obligations of each RSMC
and its network and as agreed by the regional associations.
1.8.3 PRACTICES FOR PICTORIAL REPRESENTATION OF INFORMATION ON
METEOROLOGICAL CHARTS AND DIAGRAMS
1.8.3.1 Scales and projections of meteorological charts
1.8.3.1.1 The following projections, as appropriate, should be used for weather charts:
(a) The stereographic projection on a plane cutting the sphere at the standard parallel
of latitude 60°;
(b) Lambert’s conformal conic projection, the cone cutting the sphere at the standard
parallels of latitude 10° and 40° or 30° and 60°;
(c) Mercator’s projection with true-scale standard parallel of latitude 22.5°.
1.8.3.1.2 The scales along the standard parallels should be as follows for weather charts:
(a) Covering the world:
1 : 40 000 000
Alternative: 1 : 60 000 000
(b) Covering a hemisphere: 1 : 40 000 000
Alternatives: 1 : 30 000 000
1 : 60 000 000
(c) Covering a large part of a hemisphere or hemispheres: 1 : 20 000 000
Alternatives: 1 : 25 000 000
1 : 30 000 000
1 : 40 000 000
(d) Covering a portion of a continent or an ocean or both 1 : 10 000 000
Alternatives: 1 : 25 000 000
1 : 20 000 000
1 : 15 000 000
1 : 7 500 000
1.8.3.1.3 The name of the projection, the scale at the standard parallels and the scales for
other latitudes should be indicated on every weather chart.
1.8.3.2 Symbols used on meteorological charts
1.8.3.2.1 A standard set of symbols and models should be used for plotting data on
meteorological charts.
1.8.3.2.2 A standard set of symbols should be used for representing analyses and forecasts
on meteorological charts.
NOTE: The symbols used for the pictorial representation of observational data, analyses and
forecasts on meteorological charts are those given in XXX of the Manual on Coders (WMONo.306).
1.8.3.3 Construction of aerological diagrams
1.8.3.3.1 Diagrams used for representation and analysis of upper-air observations of
pressure, temperature and humidity should:
(a) Be constructed on the basis of:
(i) The values of the physical constants and parameters given in the Technical
Regulations (WMO-No. 49), Volume I, Appendix A;
(ii) The assumption of ideal gas properties, except for the values of both
saturation vapour pressure and heats of transformation of phases of water, at
specific temperatures;
(b) Bear a legend stating the principles used in their construction.
1.8.3.3.2 Diagrams used for the accurate computation of geopotential from upper-air
observations of pressure, temperature and humidity should possess the following features:
(a) Equal-area transformation of pressure-volume diagram;
(b) Straight and parallel isobars;
(c) A scale such that the errors involved in computation are significantly smaller than
those arising from instrumental errors.
1.8.3.4 Preparation of charts and diagrams for facsimile transmission
1.8.3.4.1 Preparation of charts
1.8.3.4.1.1 When preparing charts for facsimile transmission, the following basic
considerations in the preparation of the original copy should be followed:
(a) The minimum line thickness should be sufficiently large to ensure clear
reproduction;
(b) Lines which are required to be reproduced uniformly should be of uniform width
and intensity;
(c) The minimum separations of detail in letters, figures, symbols, etc. should be
sufficient to avoid filling-in of the spaces in the reproduction;
(d) Letters, figures, symbols, etc. should be drawn as simply as possible;
(e) Models employed in plotting should be as simple as possible.
1.8.3.4.2 Standardization of maps for facsimile transmission
1.8.3.4.2.1 The standard projections and scales in paragraphs 4.1.1 and 4.1.2 should also
apply to documents prepared for facsimile transmission.
1.8.3.4.3 Colours and features
1.8.3.4.3.1 Since the reproduced chart or diagram may show little, if any, colour
differentiation between the different elements plotted on the original copy, the original should
be prepared either using a monochromatic system or, if a polychromatic system is employed,
in such a way that the reproduction conforms to a monochromatic system. For example, on
the original copy, fronts should be entered in their appropriate colours, providing the symbols
used to draw the fronts conform to the frontal symbols of the monochromatic system given in
XXX of the Manual on Coders (WMO-No.306).
1.8.3.4.3.2 Synoptic weather maps and charts prepared for transmission by facsimile should
include the following features:
(a) Geographical outlines of minimum detail necessary for orientation purposes with
coastlines interrupted where station data are to be plotted;
(b) Selected meridians and parallels printed in double thickness (bold-face) for
orientation purposes;
(c) Map references required only for convenience in the entering of data, e.g. index
numbers, 1° intersections of latitude and longitude, station circles, etc., to be printed
in non-photo blue;
(d) Letter and figures of a size compatible with resolution characteristics of the
transmission system(s) over which the charts are to be transmitted.
1.8.3.4.4 Legend
1.8.3.4.4.1 All charts and diagrams transmitted by facsimile should bear a bold legend
including:
(a) The type of chart or diagram;
(b) The date and time to which the data refer or, in the case of forecast charts, the
time to which the forecast applies;
(c) An explanation of the plotted symbols or isopleths if these are not obvious from
the style of the chart.
NOTE: Minimum requirements for identification of charts transmitted in pictorial form
are also given in the Manual on the Global Telecommunication System (WMO-No.
386) (see Volume I, Part II, paragraph 3.1).
1.8.3.4.5 Plotted data
1.8.3.4.5.1 Entries on the original copy should conform to the basic principles outlined in XXX
of the Manual on Coders (WMO-No.306).
1.8.3.4.6 Analysed data
1.8.3.4.6.1 Isopleths, frontal symbols, areas of precipitation, etc. should be entered, as
appropriate, in the manner laid down in XXX of the Manual on Coders (WMO-No.306). Care
should be taken not to obliterate one set of plotted data by another.